Process of preparing wet strength paper containing regenerated cellulose formed in situ therein



United States Patent 3,475,270 PROCESS OF PREPARING WET STRENGTH PAPER CONTAINING REGENERATED CEL- LULOSE FORMED IN SITU THEREIN Mamerto M. Cruz, Pennington, N.J., assignor to FMC Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Oct. 24, 1966, Ser. No. 588,740 Int. Cl. D21h 3/24; D21d 3/00 US. Cl. 162-177 2 Claims ABSTRACT OF THE DISCLOSURE Wet-laid paper of increased wet strength and improved properties is obtained by absorbing cellulose xanthide on the fibers of said wet-laid paper, heating the thus treated wet-laid paper at 120 to 350 F. under alkaline conditions to convert the cellulose xanthide to cellulose Xanthate, contacting the resulting wet-laid paper with an aqueous acid having a pH of from 1 to 4 and converting said cellulose Xanthate to regenerated cellulose to bind the fibers of the wet-laid paper.

wherein R0 refers to B-anhydroglucose units. It is made in a known manner by reaction of cellulose with CS and caustic soda. The water-soluble cellulose Xanthate may be introduced onto the pulp fibers by either of two methods. In the first method the cellulose Xanthate is used as an internal sizing additive and is introduced into an aqueous pulp suspension. Thereafter, when the suspension of pulp is formed into paper, a portion of cellulose Xanthate remains on the cellulosic fibers of the papers. In the other method a preformed paper sheet is passed through a water solution of cellulose Xanthate and the Xanthate is absorbed onto the paper sheet.

The wet paper treated by either of the above methods contains residual absorbed cellulose xanthate; this is converted to regenerated cellulose by passing the paper through an acid bath. The resulting film of cellulose, formed in situ, deposits throughout the paper sheet. It bonds the cellulosic fibers of the paper at their points of contact and interlocks the fibers in place thereby increasing the wet strength of the paper.

One serious problem that has arisen in the use of cellulose xanthates as a paper additive for conversion to regenerated cellulose is the extreme poor stability of these compounds. This necessitates producing the xanthates on demand just prior to being used. The cellulose xanthates cannot be produced in one location and then stored until ready for use in another location. This is inconvenient and makes operation diflicult because the production of the cellulose Xanthate must be varied to coincide with the fluctuating demands for the final paper product.

Another difficulty in using a commercial cellulose xanthate solution as a paper additive is that sulfur side-products produced during the manufacture of cellulose xanthate are retained on the paper. These side products, e.g.,

Patented Oct. 28, 1969 Na S, Na SO and Na C have a strong odor per se or break down into odorous compounds that adhere to the paper even after copious washing. These compounds are sufiiciently water soluble to preclude being separated readily from water solutions of cellulose Xanthate. Further, even small amounts of these residual, sulfur sideproducts impart an undesirable odor to the final paper.

As a result there is a need for a stable paper additive which will increase the strength of paper and which has sufiicient stability to enable it to be produced and stored in a dry form without deteriorating prior to being used.

It is an object of the present invention to satisfy the above need.

It is a further object of the present invention to utilize a compound as a paper additive which is highly stable, which can be produced. from cellulose, and which leaves no undesirable odorous sulfur impurities.

I have now found that the Wet strength of wet-laid papers can be increased and their physical properties otherwise improved by absorbing cellulose xanthide on the cellulose fibers of a wet-laid porous paper in amounts of from about .05 to about 12% (preferably about 1 to 8%), heating the wet-laid paper containing said cellulose xanthide under alkaline conditions to a temperature of 120 to 350 F., and subsequently acidifying said wet-laid sheet by contact with an aqueous acid solution having a pH of l to 4 and recovering a porous wet-laid paper whose pulp fibers are bonded together by regenerated cellulose and which is substantially free of any odorous sulfur impurities.

In order to carry out the present invention cellulose xanthide must be formed from cellulose. This is achieved by treating a cellulose product derived from cotton, wood, etc. with sodium hydroxide and carbon bislufide to produce cellulose Xanthate, and subsequently oxidizing the Xanthate to the xanthide, as shown in the following reaction.

agent where RO refers to B-anhydroglucose units of cellulose.

In this reaction the sodium hydroxide and carbon bisulfide are added to the cellulose in mole ratios, respectively, of at least 0.33:0.15 per mole of cellulose. The reaction is carried out at temperatures of about 10 to about 60 C. in a sealed chamber 'which prevents the escape of carbon bisulfide. The resulting cellulose Xanthate has a DS (degree of substitution) of at least about 0.02. The DS expresses numerically the moles of carbon bisulfide that have reacted with one mole (162 grams) of cellulose. The resulting cellulose Xanthate is then oxidized to cellulose xanthide by means of oxidizing agents such as iodine or hypochlorite. In practice this is carried out by first acidifying the cellulose Xanthate with an acid such as sulfuric, acetic, hydrochloric or any other inorganic or organic acid to a pH not lower than about 5.5 and then adding the desired oxidizing agent, e.g. the sodium hypochlorite solution, in an amount sufiicient to convert the cellulose Xanthate to cellulose xanthide. An excess of hypochlorite solution is normally used over the theoretical quantity required for conversion of the cellulose Xanthate groups to the cellulose xanthide groups.

Normally a molar ratio of sodium hypochlorite to the Xanthate group of 1.5:1 to 1.9:1 has been found satisfactory in producing the cellulose xanthide. In this oxidation step the cellulose Xanthate solution is normally maintained at a pH of between 5.5 and 7.5 and preferably at a temperature of 50 to F. to avoid decomposing the Xanthate groups. The resultant cellulose xanthide product is recovered as a substantially pure, whitish, water-insoluble powder, which has a much greater stability than cellulose xanthate.

The production of cellulose derivatives is well known by those skilled in the art and the above-defined method is not intended to limit the source or the method of producing cellulose xanthide which is used in the present invention.

In accordance with the present invention cellulose xanthide is applied to the cellulose fibers of porous papers and absorbed therein. The paper is then heated under conditions which will permit the cellulose xanthide to be converted to cellulose xanthate. The paper is then contacted with an aqueous acid solution to convert the cellulose xanthate to regenerated cellulose which bonds the cellulosic fibers together and imparts wet strength to the resultant paper.

More specifically the preferred manner of carrying out the present invention is by passing a wet-laid, highly porous paper through an aqueous, alkaline, colloidal dispersion of cellulose xanthide. The wet-laid papers which are treated in accordance with this process are those having relatively long cellulose fibers, i.e., at least 4 inch long, such as those commonly used in the manufacture of highly porous papers, e.g. filter paper. The resultant paper wetted with the cellulose xanthide dispersion is then passed between rollers to remove excess amounts of the xanthide dispersion. The take up of cellulose xanthide in the paper is controlled to assure that at least effective amounts remain on the paper for subsequent processing.

The thus treated wet-laid paper is then passed through a heating zone maintained at a temperature of about 120 to 350 F. for a suflicient period to convert the cellulose xanthide to cellulose xanthate. This normally requires about /2 to 5 minutes to obtain the desired conversion. The presence of an alkaline residue aids in effecting conversion of the xanthide group to the xanthate form. The more alkaline the paper during this heating step, the more rapid is the conversion of the xanthide to the xanthate.

The resultant paper is then passed into an aqueous acid solution having a pH of from about 1 to about 4. The particular acid utilized is not critical except that it must not attack the cellulosic fibers per se, nor degrade the paper in any way. Suitable acids which may be utilized include sulfuric, acetic, hydrochloric acid, and other known organic or inorganic equivalents thereof. Acid salts such as NaHSO NaH PO also may be used to make up the aqueous acid solution.

Upon contacting the treated fibers of the paper with the above aqueous acid solution, the absorbed cellulose xanthate is converted to regenerated cellulose in situ. The resultant regenerated cellulose is in a swollen form and deposits throughout the paper sheet. It coats the cellulosic fibers and bonds them at their point of intersection. The resultant cellulose impregnated paper is then washed of any residual acid and dried. During the drying operation the regenerated cellulose shrinks and interlocks the long woven cellulose fibers of the paper in place, thereby increasing the papers wet strength.

As an alternate embodiment of the cellulose xanthide may be applied to the cellulose fibers of a wet-laid sheet by adding an alkaline dispersion of cellulose xanthide directly to the pulp suspension as an internal sizing agent. In this embodiment the required amount of cellulose xanthide is added to water along with an alkali e.g. NaOH and finely dispersed to form a colloidal dispersion. This dispersion is added to a conventional pulp slurry of relatively long length cellulose fibers useful in making highly porous papers. The pulp slurry is then used to make up wet-laid sheets of paper by passing the slurry onto a fine screen in a conventional fashion. On a laboratory scale this is carried out by placing the above pulp slurry in a box whose bottom is made up of a fine screen. The slurry then is permitted to pass onto the screen. The cellulose pulp fibers and cellulose xanthide particles remain on the Screen to form a wet-laid sheet while the water passes therethrough. Thereafter the wet-laid sheet may be treated as set forth above to convert the cellulose xanthide to regenerated cellulose by an initial heating step followed by acidification of the paper.

The regenerated cellulose deposited on the paper in the instant process is substantially free of odor-forming, sulfur impurities which are normally encountered when producing regenerated cellulose from cellulose xanthate. This is because the ultimate precursor of the regenerated cellulose, namely cellulose xanthide, can be recovered free of these substantially water-soluble impurities during its manufacture. More specifically, during the oxidation step when an aqueous solution of cellulose xanthate is oxidized to cellulose xanthide, the xanthide precipitates substantially free of such sulfur impurities. The sulfur impurities either remain in solution, or are oxidized or volatilized into forms which do not precipitate with the cellulose xanthide. Since the cellulose xanthide precursor is free of sulfur impurities, the intermediate derivative, cellulose xanthate and the final derivative, regenerated cellulose, can be produced substantially free of these odorous, sulfur impurities.

It should be understood that the manner of applying the cellulose xanthide to the cellulose fibers of the paper is not critical so long as the xanthide is applied uniformly and in the amounts specified. Further, any technique useful in applying the cellulose xanthide to the wet-laid sheet may be utilized without departing from the scope of the present invention.

The following examples are given to illustrate the invention and are not deemed to be limiting thereof.

EXAMPLE A Preparation of cellulose xanthide A freshly prepared cellulose xanthate solution weighing 1812 grams and containing the equivalent of 9% cellulose and 6% NaOH and having 30.5% CS (based on the weight of the cellulose) combined therewith was mixed with deionized water having a temperature of 10 C. in a polyethylene container. The amount of water added was sufficient to give a solution having a concentration of 4.5% of cellulose equivalent. The solution was acidified by dropwise addition of 100 ml. of 10% acetic acid solution until a pH of 6 to 8 was obtained.

An alkaline sodium hypochlorite solution having a volume of 1390 ml. and having an equivalent of 73 grams of NaOCl was partially neutralized to a pH of 7.5 with a dilute acetic acid solution. The resultant solution was added to the dilute xanthate solution with vigorous agitation. The resulting mixture was maintained at a temperature of below about 20 C. during the ensuing reaction.

When about half the required hypochlorite solution was added, gelation occurred. Upon further addition of the hypochlorite solution, a curd-like precipitate formed which broke into discrete particles. The precipitate was filtered from the mother liquor and washed in deionized water until free of salts. A yellow product was obtained containing 4.08% sulfur and 10.1% moisture. The product was identified as cellulose xanthide and weighed 171 grams on a dry basis.

EXAMPLE 1 Run A.Twenty-two grams of 30 mesh cellulose xanthide powder containing 4.08% sulfur and 10.1% moisture was moistened with 25 ml. of ice cold water and was allowed to stand for 5-10 minutes. One hundred and eighty grams of ice cold 10% NaOH solution was added to the' moist xanthide. After 5-10 minutes of stirring a homogeneous solution of the cellulose xanthide was formed. This mixture was diluted with water having a temperature of 10 C. and agitated to yield a final mixture containing 2% cellulose xanthide and 1.8% NaOH.

Dry paper hand sheets produced from a mixture of 75% hemp and 25% soft wood, sulfite pulp and having a 10 pound basis weight (24 x 36-500) were treated with the above dilute xanthide mixture in accordance with the following laboratory tube-sizing procedure:

The treated sheets were tested as set forth above and the results are shown in Table 1.

TABLE 1 Breaking Paper Length, m. Burst Factor Tear factor Sample Paper Make Up Additive Amount Cond. Wet 00nd. Wet Cond. Wet

The untreated paper handsheet was placed between two Noble and Wood 100 mesh screens and immersed in the cellulose xanthide mixture for 20 seconds. Thereafter the treated sheet, still held between the two screens, was pressed between the rollers of a wringer.- The pressed sheet, still held between the two screens was dried on a Noble and Wood hotplate at 325 F. for one minute. Thereafter the dried sheet was dipped in a cold sulfuric acid solution for 30 seconds and the excess acid was washed out by passing tap water through the sheet. Traces of residual acid in the sheet were removed by soaking the sheet in 2% ammonium hydroxide solution for 10 seconds and then by water washing the sheet in running water until neutral. The neutral wet sheet was. then placed between two pieces of blotting paper, pressed between two Noble and Wood screens, and dried for 2 minutes on a Noble and Wood hotplate at 325 F. The dried sheets were then conditioned and tested in accordance with Tappi Standard 220m60. The percent cellulose xanthide pick up was determined by averaging the weights of five sheets before and after treatment. The results of the testing are set forth in Table 1. The handsheets had no sulfur odor.

Run B.The procedure of Run A was repeated using a typical soft wood, unbleached Kraft pulp (Canadian Standard Freeness 660) to produce dry paper handsheets instead of the mixture of hemp and soft wood sulfite pulp.

What is claimed is:

1. The process of increasing the wet strength of wetlaid papers which comprises adding cellulose xanthide having a DS of at least 0.02 to the cellulose fibers of a wet-laid porous paper in amounts of from about 0.5 to about 12% based on the weight of the pulp, heating the wet-laid paper containing said cellulose xanthide under alkaline conditions to a temperature of about to about 350 F. for a time suflicient to convert said cellulose xanthide to cellulose xanthate, subsequently contacting the heat-treated wet-laid paper with an aqueous acid solution having a pH of about 1 to about 4 for a period sufiicient to convert the cellulose xanthate on said wetlaid paper to regenerated cellulose and recovering a porous wet-laid paper whose cellulose fibers are bonded together by regenerated cellulose and which is substantially free of odorous sulfur impurities.

2. Process of claim 1 wherein said cellulose xanthide is present in amounts of from about 1 to about 8%, based on the weight of the dry pulp.

References Cited UNITED STATES PATENTS 1,745,557 2/1930 Richter et a1 117157 X 2,422,573 6/ 1947 Lilienfeld.

3,304,223 2/ 1967 Wheeler 162177 X US. Cl. X.R. 

